As is well known in the art, heat exchangers, such as plate and fin heat exchangers, typically include separate hot and cold circuits that respectively direct a working fluid and a heat-exchange fluid through adjacent hot and cold layers. The hot and cold layers are often stacked on top of each other to form a core of the heat exchanger. Each layer may have one or more heat-transfer fins positioned within the layer in a ruffled, serrated or serpentine manner to maximize the surface area of the fins within the layer. The heat-transfer fins are affixed to parting sheets at opposed surfaces of the layers. The sheets separate the layers from each other and form barriers to prohibit mixing of the fluids in adjacent hot and cold layers.
In operation of a plate and fin heat exchanger that is being used to extract heat from the working fluid, the working fluid passes through a hot circuit that directs the fluid through at least one hot layer, while the heat-exchange fluid is directed by a cold circuit through an adjacent cold layer. Heat from the working fluid moves through the heat-transfer fins of the hot layer; through the parting sheet affixed to those fins; into the heat-transfer fins of the adjacent cold layer; and, into the heat-exchange fluid to be removed from the heat exchanger as the heat-exchange fluid moves through the cold circuit out of the heat exchanger. In many plate and fin heat exchangers, the hot and cold circuits will direct the working and heat-exchange fluids through a plurality of adjacent hot and cold layers; the actual number of layers being a function of the operating and desired temperature of the working fluid, the temperature of the heat-exchange fluid, the flow rates of the respective fluids, and the surface areas of the heat transfer fins and layers.
The working and heat-exchange fluids in such heat exchangers may both be liquid, or they may both be gas, or one may be a gas while the other is a liquid. For example, in a conventional automobile powered by a liquid-cooled internal combustion engine, the radiator for the engine coolant is a standard heat exchanger wherein the working fluid is a liquid (the coolant) and the heat-exchange fluid is a gas, namely--the atmosphere. In a modern aircraft powered by a gas turbine engine, it is common to use air bled from compressor stages of the engine for many aircraft sub-systems, including cabin air conditioning. Plate and fin heat exchangers utilizing a gaseous heat-exchange fluid are frequently used in such aircraft to regulate the temperature of the working fluid which in such an example would be the compressed air bled from the engine.
In most working environments of heat exchangers, as in those described above, a critical design parameter is a desire to reduce the weight of the exchanger as much as possible. Consequently, aluminum is almost invariably used to form the parting sheets and heat-transfer fins because of its light weight. Aluminum, however, presents significant problems in typical heat exchanger applications, especially when used as the parting sheet. Aluminum is very susceptible to corrosion. Penetration of a parting sheet as a result of corrosion may produce a pin hole leak through the sheet such that the working and heat-exchange fluids mix. In that event, the heat exchanger core must be taken out of service and repaired or discarded and replaced. Additionally, aluminum, in comparison to the metals typically used as frames, housings and/or mounting fixtures for heat exchanger cores, has a very high coefficient of expansion. Consequently, aluminum parting sheets are subject to severe thermal fatigue stress as temperatures fluctuate during use, limiting the duration of their useful life. Both susceptibility to corrosion and thermal fatigue stress therefore present substantial reliability and cost problems for known heat exchangers.
Accordingly, it is the general object of the present invention to provide an improved parting sheet that overcomes the reliability and cost problems of the prior art.
It is a more specific object to provide an improved parting sheet for heat exchangers that minimizes corrosion induced puncture of the sheet.
It is yet another object to provide an improved parting sheet for heat exchangers that minimizes thermal fatigue stress of the sheet.
The above and other advantages of this invention will become more readily apparent when the following description is read in conjunction with the accompanying drawings.